The knee joint provides flexing of the leg where a lower bone called the tibia meets an upper bone called the femur. At the knee, the femur has two projections known as femoral condyles that engage with the articular cartilage at the upper end of the tibia. Degeneration of the cartilage contact surfaces on the tibia and the femur can cause pain, swelling and limited range of motion. During total knee arthroplasty, i.e., replacement surgery, the two surfaces joining at the knee are replaced with a prosthesis. The first step in this process is the removal of some of the diseased cartilage and underlying bone that is replaced by the prosthesis with its synthetic materials. The distal end of the femur is cut in order to provide clearance for a femoral component of the knee prosthesis that provides a new surface at the distal end of the femur.
A typical femoral prosthetic component fits over the distal end of the femur and curves over the anterior and posterior portions of the femur near the distal end. Preparation of the bone for the placement of most femoral components involves a cut or resection at the distal end, a cut along anterior and posterior sides of the femur near the distal end and two angled (chamfer) cuts joining the distal end with the anterior and posterior sides respectively. A variety of approaches exist for making these cuts that may involve making the cuts in different orders. The common features of these methods is that a selection of the proper sized prosthetic femoral component must be made and the cuts must be properly placed for the selected component size.
Often, a hole is drilled into the center of the femur from the distal end. An intermedullary rod is then placed within this hole to provide a relatively reproducible base from which implant components are oriented and located within the joint. In one approach, an adjustable block is attached to the intermedullary rod to guide the distal femoral bone cut. The cut distal surface can then provide a base for the size measurements and the remaining cuts.
After the distal cut is performed, a sizing jig is used to select the proper sized femoral component based on the size of the bone. The sizing jig serves as a guide for the drilling of holes later to be used for attaching a cutting guide. The sizing jig may have two paddles for contacting the femoral condyles on the posterior femur and an anterior probe for contacting the anterior femoral cortex, which appears externally as a shallow groove flanked on both sides by ridges of bone on the anterior side of the femur. Typically, there is a separate cutting guide for each available size of prosthesis. The proper cutting guide is then set in place by inserting pegs on the cutting guide into the holes mentioned above, and the cuts are made. Either a single cutting guide is used for the anterior cut, the posterior cut and the two chamfer cuts, or multiple cutting guides can be used. A large number of sterilized cutting guides are needed for each surgery because the size needed is not known with certainty before the surgery begins.
Generally, the anterior cut should be made so as not to violate the anterior cortex, otherwise a serious weakening of the femoral bone could take place creating significant risk of fracture. However, it can be difficult to perform the cuts without damaging the anterior cortex when the bone size is between available prosthesis sizes. The surgeon must use great care and judgment in aligning the cut, or, alternately, multiple cuts are required to progress to the desired final surface.
The cuts can be made with a small amount of rotation relative to the plane between the natural condyles. Such rotation reduces the chance of dislocation of the knee cap (patella). The rotation can be obtained either by using a cutting guide placed at the angle or by using a straight cutting guide and rotating the placement of the holes for the anchoring pegs. In principle, the rotation can be designed into the final prosthesis. In any case, performing the cuts to obtain the proper orientation can be very difficult if the cartilage on the posterior femoral condyles is unevenly worn because the posterior paddles of the sizing jig abut against the two unevenly worn posterior femoral condyles.
U.S. Pat. No. 4,892,093 (Zarnowski et al.) discloses a cutting guide for making the anterior femoral cut, the posterior femoral cut, the anterior chamfer cut and the posterior chamfer cut. The cutting guide has two posts which fit into corresponding holes drilled in the distal end of the femur, and the guide is further secured with a screw inserted into the distal end of the femur. The post holes must be positioned properly before the cutting guide is used. Using a single cutting guide for all four cuts provides a more accurate relationship between the different cuts while reducing the time required for the cuts. However, the cutting guide does not provide any flexibility in the placement of the cuts, so a different cutting guide must be used for cuts corresponding to each prosthesis size.
U.S. Pat. No. 4,759,350 (Dunn et al.) describes a system of instruments for preparing the distal femur and proximal tibia for receiving components of the knee prosthesis. The system uses the femoral intermedullary canal as a reference for the femur cuts. The system is designed to make a first cut of the femur along the anterior face of the distal end of the femur. The second cut is at the distal cut on the distal end of the femur. A gauge is used to select from six sizes for the eventual prosthesis component for the femur. A finishing cutting guide is selected based on the size read from the gauge, and the finishing cutting guide is used to perform a posterior cut, two chamfer cuts and a second anterior cut. The system described requires that two anterior cuts are ultimately performed to obtain the proper distal surfaces for the placement of the prosthesis. The cutting guide has a ledge for contacting the cut anterior surface to prevent motion of the cutting guide. The guide also has two pegs which fit into holes drilled into the distal end of the femur.
U.S. Pat. No. 4,938,762 (Wehrli) and U.S. Pat. No. 4,567,885 (Androphy) involve resection systems where a cutting block is transferred to guide cuts on both the tibia and the femur. These systems have the disadvantage that the cutting block either has to be selected based on the size of the prosthesis component or they must be explicitly adjusted for each cut to accommodate the different size prostheses. These systems are difficult to use and leave considerable room for error.
U.S. Pat. No. 5,002,547 (Poggie et al.) describes a modular apparatus for preparing both the tibia and femur for a knee prosthesis. The distal end of the femur is the first surface of the femur that is cut. A sizer is placed against the cut surface at the distal end to determine the correct size for the prosthesis. Then, a guide is placed against the cut distal end of the femur, and is used for drilling holes based on the selected size for the prosthesis component. The guide has a set of holes for each size component. An appropriately sized cutting guide is selected. The cutting guide is used to perform the anterior cut, the posterior cut and the two chamfer cuts.
U.S. Pat. No. 4,457,307 (Stillwell) describes a complicated device for performing all needed cuts on the tibia and the femur. The device includes a power saw and all the adjustments needed to align the saw. While this instrument is very versatile, it is very awkward and difficult to use. Bolts must be loosened to position the instruments, and then tightened in order to make the cuts. Furthermore, while the instrument can be adjusted to accommodate different size prosthesis components, calculations must be performed in order to make the adjustments.
U.S. Pat. No. 4,703,751 (Pohl) presents a cutting guide for cutting the distal end of the femur as a first step of preparing the bone for a prosthesis component. The resection guide provides for severe condylar deficiency. The cutting guide has a cutting plate that pivots relative to a support plate. The cutting plate locks relative to the support plate at a predetermined angle. The support plate attaches to an intramedullary rod for alignment of the cut. A reference bar pivots relative to the rod and contacts both the lateral and medial condyles. A plurality of reference bars are selected to provide the appropriate thickness of the cut to avoid cutting excessive material from the condyles. Bolts within the reference bar enable the surgeon to fine tune the cutting distance and similarly to make it easier to compensate for the particular deterioration of one of the condyles. There is no suggestion of how to adapt the apparatus to make other cuts on the femur.
U.S. Pat. No. 4,722,330 (Russell et al.) discloses an apparatus which attaches to an intramedullary alignment rod for providing an anterior cut, a posterior cut and two chamfer cuts. The apparatus has a main body which is attached and aligned once. The other parts are attached to the main body as needed. The apparatus has an adjustment relative to the anterior cortex to provide for proper location of the cuts on the anterior and posterior surfaces. The main body has a fixed relationship between the cutting guides for the different cuts. Therefore, a different main body would be needed to make cuts for different size femoral prosthesis components.
U.S. Pat. No. 5,364,401 (Ferrante et al.) describes a system for resecting the femur where various cutting guides can be sequentially placed on a support that is initially positioned on the bone. The final step in the preparation uses a cutting block that can make an anterior cut, a posterior cut and two chamfer cuts. The cutting block can be moved along the cut distal end of the femur relative to an alignment portion attached to the anterior part of the femur. This structure, which allows motion, aides in aligning the cuts to avoid cutting too much bone from the anterior surface. The cutting block must be selected to correspond to a selected size prosthesis component.
The above described instruments for preparing the distal end of the femur are all deficient in that they are either difficult to use, not versatile in their placement or both.